The “airlift method” is understood as the method for transporting removed sea bed. The airlift method provides a supply of compressed air into the bottom area of the conveying line. The air bubbles that rise on the inside of the conveying line create the effect of an upward flow on the inside of the drilling line that transports removed sea bed to a marine unit above the water line.
When such a conveying apparatus is employed for transporting mineral raw materials, such as, for example, manganese nodules from a water depth of approximately 5,000 m, the volume portion of the material transported inside the conveying line can constitute up to 10% of the internal volume of the conveying line. The conveying line can, for example, have an inside diameter of 40 cm.
It is regularly possible to generate a stronger upward flow if feed pumps are used. The volume fraction of the conveyed material is then greater, however, the method tends to be even more susceptible to clogging.
If the conveying operation of removed sea bed comes to a standstill (irrespective of the reason therefor), the sea bed material that is inside the conveying line sinks very quickly to the bottom because it has a considerably higher density than sea water. Assuming a water depth of 5,000 m and a volume fraction of removed sea bed of 10%, the result is a 500 m long plug clogging the line. Freeing the conveying line of the plug by regular means is then either impossible, or only possible with great difficulty. Similarly, it is no longer possible to salvage the conveying line due to the large mass of the plug, which can be as much as 1,500 to 2,000 τ in the given example. In a worst case scenario, this means that the conveying line may need to be abandoned following such an interruption of the conveying operation.
A reason for such an interruption can be, for example, a failure of a transport of flow inside of the conveying line. Such a failure can be caused by deposits of removed sea bed on the interior lining of the conveying line which gradually increase until they create a blockage of the complete internal cross-section or of the conveying line. Another conceivable reason for a blockage is an energy supply failure or a compressor failure which results in the compressed air necessary for the operation of the airlift process no longer blowing into the conveying line. If the sea bed is first pumped via solid-material pumps from a clearing vehicle to an interim station, which is also referred to as a “buffer,” and transported from there via the conveying line to the marine unit above the water line, defects on the submarine unit can also result in a failure of flow transport. Extreme environmental events having a propensity of causing an interruption in flow transport are moreover conceivable.
DE 2008384 A describes a dual pipe conveying facility that has an annular pipe line with pipes that are routed as a sink pipe from the ocean surface down to the ocean floor and as a lift pipe for the transported material back up to the ocean surface. Pressurized water preferably circulates inside this annular pipe line as a transport fluid, wherein the pressurized water is circulated by pumps. The conveyed material is fed into the annular pipe line via a pressure lock on the ocean floor. The pressure of the pressurized fluid is dimensioned such that the conveyed material fed into the annular line is raised inside the lift pipe all the way to the water surface.